Background Paper prepared for the Global Assessment Report on Disaster Risk Reduction 2013

26 Abstract The present paper would focus on lessons that have been learnt through field trips carried out in southern Vietnam and numerical analysis that will show how the lack of knowl

Coastal Disaster Risk in Southern Vietnam The Problems of Coastal Development and the Need for Better Coastal

Planning

Hiroshi Takagi

Tokyo Institute of Technology

Nguyen Danh Thao

Ho Chi Minh City University of

Technology

Miguel Esteban

Waseda University

Tran Thu Tam

Ho Chi Minh City University of

Background Paper prepared for the Global Assessment Report on

Disaster Risk Reduction 2013

Geneva, Switzerland, 2013

Coastal Disaster Risk in Southern Vietnam

－the Problems of Coastal Development and the Need for Better Coastal Planning－

Hiroshi Takagi 1 , Nguyen Danh Thao 2 , Miguel Esteban 3 , Tran Thu Tam 2 ,

Hanne Louise Knaepen 4 , Takahito Mikami 3 and Lilian Yamamoto 5

List of Contents

1 Introduction 2

2 Analysis of natural hazards causing coastal disasters 3

2.1 Tropical cyclones 3

2.2 Storm surges 4

2.3 Tsunamis 7

2.4 Coastal erosion 12

2.5 Topographical hazard 18

2.6 Sea-level rise 19

3 Discussion on coastal disaster vulnerability 20

4 Conclusions 23

References 24

Appendix 26

Methodology of storm surge simulation 26

Abstract

The present paper would focus on lessons that have been learnt through field trips carried out in southern Vietnam and numerical analysis that will show how the lack of knowledge to understand either future environmental impacts or strong regulations are leading to increased risk in many coastal areas The authors attempted to analyse the potential disaster risks associated with six natural hazards: tropical cyclones, storm surges, tsunamis, coastal erosion, topographical hazard and sea-level rise, and discussed the vulnerability of local communities to these threats in the context of rapid economic development The authors pointed out that infrastructure investments made by tourist or other industries may exacerbate the potential disaster risks on adjacent areas, and the potential risks due to coastal disasters can become larger among the poorest members of the community, who often live in higher risk areas as their adaptive capacity and resilience it typically lower than richer members of the society

1 Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8550, Japan

2 Ho Chi Minh City University of Technology, 268 Ly Thuong Kiet St., Dist.10, Ho Chi Minh, Vietnam

3 Waseda University, Ookubo, Shinjuku-ku, Tokyo, 169-8555, Japan

4 Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto, 606-8501, Japan

5 UNU-IAS, Pacifico-Yokohama 1-1-1 Minato Mirai, Nishi-ku Yokohama, 220-8502, Japan

The present paper will focus on lessons that have been learnt through field trips carried out in Vietnam and numerical analysis that will show how the lack of knowledge to comprehend future possible environmental impacts and the absence of strong regulations are leading to increased risk in many coastal areas Private investment, such as the building of jetties by hotel owners, can cause significant coastal erosion in other areas, often inhabited by poorer members of the community This coastal erosion can exacerbate the risk due to natural disasters such as typhoons or (rare for the case

of southern Vietnam) tsunamis, as a result of losing the protection offered by natural barriers These events have been documented by the authors in their own research in Vietnam, and will be analyzed within the current climate change discourse

The authors will bring all these issues into perspective by analyzing the current thinking of academics, policy makers and civil servants in Vietnam and what are currently considered to be the best way to optimize risk management in various types of Vietnamese coastal environment on the basis of natural and social science approaches In fact, the factors leading to the vulnerability of the

coast are quite complicated, as shown in Fig.1 In the present study, the vulnerability of coastal areas

against coastal disasters has been discussed in the context of the typical model of a rapid development of the coastal zones, such as that of Vietnam at present

-2-Fig.1 Factors leading to the vulnerability of the coas t

2 Analys is of natu ral hazards causing coastal disasters

The present paper dis cusses the ulnerabilit y of coasta areas in s uthern Vietnam, and analyses th e risks associated with tropical cy clone stor m surges, tsunamis a d other po tential ris s based o n numerical imulations and findin s from field surveys carried out by the authors

2.1 Tropic al cyclones

The peak occurrence o f typhoon andfalls In Vietnam is normally d uring the m onth of O tober in th

e central region and No vember in the South There hav ing been roughly around 786 typhoons an d tropical st rms that a pproached or affecte d Vietnam during the 20th cent ury, of which 348 ar e typhoons ith wind speeds gre ter than 120 km/h hese storm s typically do hit the mainland, especially the coastal provinces in the Nor th and the Centre of Vietnam ( Kleinen, 2 007) In th e present study, the so-called Best Track Da ta, obtained from The Joint Ty phoon Warning Center (JTWC), is used to analyze the typhoon tr acks aroun d the East Sea (South China Se ) The dat a consists of time, geographical po sition, min imum sea level press ure and maximum sustained win d

speed in k nots of eac h typhoon hroughout its life Fi 2.1 shows more than 200 track s of tropical

storm which developed in the East Sea and approache Vietnam’s coasts in the past 60 years (from

1951 to 2010) The figures were separated into decades in order to identify any trends in the occurrence of tropical storms In the figures, a diamond shape symbol is displayed when the tropical storm becomes a typhoon, in which the wind speed is greater than 64 knots (32.7 m/s) It appears that typhoons or tropical storms are much less frequent in the southern than in the northern and central parts of Vietnam This is mainly due to the reason that the Coriolis effect, which initiates and maintains tropical storm rotation, is weaker in lower latitude Nonetheless it appears that the number

of tropical storms that hit southern Vietnam is noticeable and non-negligible, even though Vietnamese people generally think the southern coast of Vietnam is free from the threat of typhoons

whereas the northern and central parts always suffer severe typhoons Table 2.1 again shows the

number of tropical cyclones that passed around Vietnam’s coasts in the last 6 decades Each tropical cyclone was categorized into two latitude zones (N25°- 15° and N15°- 5°) according to point of where it disappeared, in order to identify trends in movement over time MONRE (2009) points out that there are more typhoons with higher intensity and that typhoon tracks have had a tendency of moving southward in recent years However, no clear evidence of this tendency can be seen from

Fig.2.1 and Table 2.1

2.2 Storm surges

Vietnam, with 3,260 km coastline, is one of the most vulnerable countries against coastal disasters, especially storm surges caused by tropical cyclones A storm surge is an increase in the level of sea water which is caused by high winds pushing on the ocean’s surface combined with the effect of low pressure at the center of a weather system Although there seems to be comparatively little research having been carried out on storm surges in the coasts of Vietnam until now, a series of studies have been made by Vietnamese researchers (i.e Pham 1992) under United Nations Development Program (UNDP) projects Pham (1992) gave an overview of storm surges in the northern coast of Vietnam from 16°N to 22°N Ngueyen (2008) also performed a simulation of Typhoon Ling Ling (which made a landfall on the central coast of Vietnam in 2001) by using sophisticated numerical models

To the authors’ knowledge, however, there are few studies that have evaluated the storm surge risk due to tropical storms to the southern part of Vietnam Part of the reason for this seems to be that tropical storms occur less frequently in the southern part of Vietnam, whereas the northern and central areas of the country are frequently affected by tropical storms (GTZ 2003) However, it is important to remember that sometimes tropical cyclones venture into the southern part of Vietnam,

as shown in Fig.2.1 The authors in the present paper thus attempt to analyze what are some of the potential dangers to some areas in southern Vietnam, and prove that disaster risk management in

these areas should consider the potential for storm surges to affect low-lying areas

-4-Fig.2.1 J TWC Typh oon Best T racks arou nd East S a from 19 51 to 2010

Table 2.1 Number of tropical storms th at approached Vietnam’s coasts from 1951 to

2010, classifying according to latitu de zone an d period

3) Estimation of storm surge height in southern Vietnam

To estimate storm surge height due to past tropical storms, a series of simulations were performed for 5

selected tropical storms that approached the southern part of Vietnam in recent decades (Fig.2.2):

Typhoon Tilda, in November-December

1954 Typhoon Lucy, in November 1962

Tropical Storm Thelma, in November 1973

Severe Tropical Storm Linda, in October-November

1997 Typhoon Muifa, in November 2004

Fig.2.2 The 5 selected tropical storms approaching the southern part of Vietnam

Using this computer simulation, the sea water elevations were calculated for two different locations,

one at Phan Thiet and the other at the Mekong River mouth Table 2.2 shows the highest storm surge

elevations calculated at each of these points for the 5 selected storms Most of the storm surge heights are less than 0.5 m, except for a 0.56 m at Phan Thiet during typhoon Lucy in 1962 and 0.70m at the Mekong River mouth during severe tropical storm Linda in 1997

Table 2.2 Calculated storm surge heights during the past tropical storms

Mouth

Note: contribution due to wave setup is not taken into account

The wave setup was calculated, for simplicity, by assuming that offshore waves propagate normal to

-6-the shorelines over a uniform slope, ei-6-ther 1/100 or 1/500, as identified by conducting a GPS

bathymetric survey carried out by the authors themselves (Fig.2.3) As the water depth becomes

shallower, wave setup becomes larger and reaches up to approximately 40 cm for Lucy and 30 cm for Linda, irrespective of slope

Finally, it is estimated that the total storm surge height (including wave setup) could be nearly 1 m for both Lucy and Linda

Fig.2.3 Scenes of bathymetric survey at Phan Thiet city in Vietnam

2.3 Tsunamis

There is a lack of understanding on the possible effects of tsunamis on the Vietnamese coast due to the limited record about past tsunami events in Vietnam However, the UN Program Coordination Group on Natural Disasters and Emergencies (2011) pointed out that part of the Vietnamese coast can potentially be affected by a tsunami, though the probability of a tsunami event is relatively low

After the 2004 Indian Ocean Tsunami, awareness on the possibility of tsunami disasters has spread to countries and areas which were generally not considered to be tsunami-prone before Tsunamis in the East Sea, which could potentially affect the Vietnamese coast, has been investigated by a number of

researchers (e.g Liu et al., 2007; Liu et al., 2009; Megawati et al., 2009; Dao et al., 2009; Okal et al.,

2011) In these investigations, certain scenarios for possible tsunamis in the East Sea were presented and discussed The Vietnamese Government also developed 25 scenarios for tsunami generations, which can be classified by the location of earthquake as follows:

 the Manila Trench (1-17) 



 the Ryukyu Trench (18 and 19) 

 the northwestern part of the East Sea and the southern part of Hainan Island (20 and 21) 



 the northern part of the Philippines and the southern part of Taiwan (22-24) 



 the western part of the East Sea and the middle part of the Vietnamese coast (25) 

Following these scenarios, the Vietnamese government has started to prepare for the future possibility of tsunamis

In the present paper, the authors carried out a numerical simulation for the possible tsunami assuming one of the worst scenarios for Vietnam’s coasts

1) Numerical simulation model

The Vietnamese coast faces the East Sea, which is surrounded by the southern part of China, Taiwan, the Philippines, Borneo, the Malay Peninsula and the Indochinese Peninsula The East Sea is connected to the East China Sea, the Pacific Ocean, the Sulu Sea, the Java Sea, and the Indian Ocean through the Taiwan Strait, the Luzon Strait, the Mindoro Strait and the Balabac Strait, the Karimata

Strait, and the Malacca Strait, respectively (Fig.2.4) These adjoining seas are connected to the East

China Sea only thorough narrow straits so that tsunamis generated in one sea do not leak into another

(Okal et al., 2011) Thus, the most important tsunamis to consider for the case of the Vietnamese

coast are those generated in the East Sea Many locations in the East Sea are recognized as having a

high potential to generate a devastating tsunami For example, Okal et al (2011) presented 14

scenarios of potential tsunamis in the East Sea and its adjoining seas Among these scenarios, a tsunami generated by an earthquake taking place in the Manila Trench is recognized as one of the

most potentially hazardous tsunamis (e.g Liu et al., 2009)

Hence, the authors carried out a numerical simulation about a tsunami generated by an earthquake taking place in the Manila Trench in order to clarify the characteristics of the tsunami on the Vietnamese coast In this simulation, the hypothetical catastrophic earthquake scenario proposed by

Okal et al (2011) is used The parameters of this fault model are summarized in Table 2.3 The

moment magnitude (Mw) is obtained from the following equation:

log 1.5 16.1

where M0 is the seismic moment in dyne centimeters (Hanks & Kanamori, 1979) The seismic moment of this model is 1.0×1029 (dyn·cm) and hence the moment magnitude is 8.6 It should be

noted that more serious scenarios have been proposed for the Manila Trench (e.g Mw = 9.0 scenario

proposed by Megawati et al (2009)) and thus the scenario considered here is still not the worst one

The governing equations of this simulation are the linear shallow water equations and a leap-frog

-8-scheme was employed to solve the equations The initial water level movement is equivalent to the displacement of the seafloor which is calculated based on the set of formulas proposed by Mansinha and Smylie (1971) Bathymetry data is obtained from the General Bathymetric Chart of the Oceans (GEBCO) organized with a grid size of 30 seconds The simulation is carried out on a 4.5 minutes

(about 8.1 km) grid extending from 10°S to 26°N and from 95°E to 131°E (Fig.2.5)

2) Estimation of tsunami height in Vietnam

Fig.2.6 shows the results of the numerical simulation The maximum amplitude is around 2m along the western coast of Luzon Island and the middle part of the Vietnamese coast (Fig.2.6 (a)) This

distribution of the maximum amplitude results from the directivity of tsunami Although the heights

of a tsunami are expected to be affected by various factors, such as refraction, diffraction and reflection by the bottom irregularities, and also by the form of a bay, the heights in the direction of the minor axis, as a rule, appear to be higher than those in the direction of the major axis of a fault (Hatori, 1963) Because the fault runs from north to south in this simulation, this results in high tsunami heights to the east and west sides of the fault The first wave arrives to the middle part of the Vietnamese coast around 2 hours after the earthquake occurs and then gradually propagates to the

northern and southern part of the coast (Fig.2.6 (b)) The area between the fault and the middle part

of the Vietnamese coast is relatively deep; meanwhile the continental shelf, where the bathymetry is shallower than 200 m, spreads off the southern part of the Vietnamese coast The velocity of the

tsunami is given by √(gh) (where g is the gravitational acceleration and h is the depth) and hence a

tsunami propagates slower in shallow area of the southern part of the Vietnamese coast and in the Gulf of Thailand

According to the results of the numerical simulation, the tsunami heights along the southern part of

the Vietnamese coast are around 1 m or less (Fig.2.7) with the first wave reaching the coast at least 2

hours after the earthquake takes place in the Manila Trench

Fig.2.4 The East Sea and surrounding seas and straits: (a) Taiwan Strait, (b) Luzon Strait,

(c) Mindoro Strait, (d) Balabac Strait, (e) Karimata Strait, (f) Malacca Strait

Table 2.3 Fault parameters (Okal et al., 2011)

Seismic moment (dyn·cm) 1.0×1029Location of the center top of fault 16.0°N, 118.5°E Depth of the top of fault (km) 10

-10-Fig.2.5 Bathymetry of the computational area and the location of the fault (given by the

red rectangle)

Fig.2.6 Results of the numerical simulation: (a) the maximum amplitude, (b) the arrival time of

the first wave

Fig.2.7 Distribution of tsunami heights along Vietnamese coast

2.4 Coastal erosion

It is feared that coastal erosion will be significantly exacerbated due to future climate-change effects such as sea-level rise and an increase in tropical cyclone intensity However, even under the present climate it is possible to find how many places throughout the world are suffering from severe erosion problems Especially, the rapidly-growing coastal cities in developing countries have been pulling in population from neighboring areas because of the various types of economic advantages they offer

However, this socio-economical development patterns often do not consider the consequences that they may have on the natural environment and how this affects disaster resilience A sandy beach can serve as a natural barrier against coastal disasters If the beach is severely eroded, the vulnerability of

local people against natural disasters will increase Fig 2.8 shows two cross-sections of the beach:

before and after erosion If the sand processes are not interfered with, then the sand profile will remain unaltered, which may reduce the energy of the waves along the shoreline, so that houses and other structures along the coast are generally protected by the sand However, if the sand is lost, bigger waves will be able to reach the coastline reach and damage houses and other infrastructure Therefore, it is important for us to understand how the beach serves as protection against natural disasters

-12-Fig 2.8 Schematic illustratio n that show s the function of a sandy beach as natural barrier

1) Field s urvey at a coastal c ity in Vietnam

Phan Thiet is the capital of Binh Thuan province in southeastern V etnam The city is known as a fast growing c astal city which had a populati on of 189, 19 as of 2009 (General Statisti s Office of Vietnam 20 12), comp ared to 75, 41 in 1979 (Thomas Brinkhoff 2012) It is l ocated clo e to the se a and is frequently visited by tour sts, as it is famous fo r its beaut ful beache s and ocea n views an d other touris m infrastru cture Phan Thiet is al so one of the most pr ductive fisheries in Vietnam wit h

50 thousand tons of fish caught per year The town provides the local comm unity with food alon g with employment and export revenues

The authors carried o t a field su rvey of this city durin g the dry season of Ja nuary 201 in order t o observe pr sent coasta l erosion problems During the s urvey, a topographical survey wa carried out

at two loca tions, Duc Long and Phu Trinh, which see m to be tw of the mo st vulnerab le locations against coastal disasters in the cit y

Fig 2.9 Depth contour off the coast of Phan Thiet (©Goo gle)

・Duc Lon g Area

Duc Long is located near a beach which has been severely eroded in recent years (Fig 2.10)

Although some of the residents have tried to stop the erosion, installing wooden piles an d sand bags, this has not been suc cessful in stopping the process A questionnaire survey conducted in Januar y

2012 by the authors w ith local r sidents in Duc Long revealed that out of 17 responden ts, 35 % ( 6 people) had to elevate their house in the past and 24 % (4 people) had to move due to coastal erosion

Using Google Earth to identify ch anges in c oastal areas, the shoreline appears to have retreated up t o 40

meters f rom 2001 t o 2010 Fi g 2.11 show s the estim ated area that was eroded during this period,

where approximately 4 ha of the sandy beac h had been lost along a 1.7 km stretch of the coastline It is also clear that a large area of the coast adj acent to Duc Long has been landfilled within this period Although a more careful assessm ent will be needed, it seems that the landfill and the t wo jetties at the river port stops the sand su pply from the river (Ca Ty Ri ver) mouth or other areas to th e downstrea m areas suc h as the Duc Long Area As a san dy beach can serve as a natural barrier against

coastal disasters such as storm surges, the situation of Duc Long clearly demonstrates how th e vulnerabili ty of coastal communities again st natural d isasters can increase if a beach is severel y eroded